In high-reliability computer disk storage systems, there is a desire to have redundancy in all the physical parts which make up a subsystem to reduce the potential for loss of data and down time upon failure of a part. The use of dual disk storage controllers, each having its own memory, provides several major benefits to a disk storage system. For example, (1) a redundancy of storage information is retained to allow for recovery in the case of failure or loss of one controller or its memory; (2) repair of a disabled controller is feasible due to the failover capabilities of the secondary controller; and (3) greater system up time is achieved through the secondary controller being available.
With the desire for more performance out of these redundant subsystems, caching and the use of memory as temporary storage has become common-place. However, the means by which these duplicate physical memories are kept in synchronization can be difficult. Some disk systems use a latent (delayed or massive update) process to create this duplication, but that approach tends to add expense, is very complex to manage, reduces performance, and limits the accuracy of recovery from failures. Another approach (the one used in this invention) is to form a real-time mirrored memory process to create and retain accuracy during the process of duplication of data. The use of real-time, synchronized, redundant memory (mirrored memory) in dual controllers can improve speed and accuracy in the case of a failover from one controller to the other.
Accordingly, objects of the present invention are to provide a dual controller disk storage system having real-time, synchronous, mirrored memory for providing immediate, accurate, and reliable failover in the event of failure of one controller or its memory.